Exploring the optoelectronic potential of K 3 AlX 6 (X=Br, I) halide compounds via first-principles calculations

Lead-free halide perovskites are promising eco-friendly materials for optoelectronic and photovoltaic applications, yet balancing stability, band gap and optical performance remains challenging. Here, density functional theory is used to systematically investigate vacancy-ordered cubic K 3 AlX 6 ([Formula: see text], I). Both compounds crystallize in the stable Fm-3m phase with negative formation energies ([Formula: see text]3.13[Formula: see text]eV/atom for Br and [Formula: see text]2.66[Formula: see text]eV/atom for I), confirming thermodynamic stability. Phonon spectra show no imaginary modes, and elastic constants satisfy Born–Huang criteria, indicating dynamical and mechanical stability. Spin–orbit-coupled calculations reveal direct [Formula: see text]-point band gaps of 3.16[Formula: see text]eV for K 3 AlBr 6 and 1.55[Formula: see text]eV for K 3 AlI 6 , demonstrating halide-driven tunability from ultraviolet to visible regions. K 3 AlBr 6 exhibits greater stiffness and higher Debye temperature, while K 3 AlI 6 shows improved ductility and lattice softness. Optical analysis indicates strong UV absorption, low reflectivity and moderate dielectric constants. Overall, halide substitution induces lattice expansion, band-gap narrowing and enhanced dielectric response, highlighting K 3 AlX 6 as stable and tunable lead-free candidates for advanced photovoltaic and photonic technologies.